A. Field of the Invention
This invention relates to a process for making solid calcium hypochlorite compositions from impure hydrated limes having relatively high amounts of magnesium contained therein. This invention also relates to making a solid calcium hypochlorite composition containing substantially water-insoluble and unchlorinated magnesium compounds which are derived from impure hydrated limes.
B. Description of the Prior Art
Solid calcium hypochlorite compositions are widely used today for water treatment, especially of swimming pools where it serves as a convenient source of chlorine for disinfectant purposes, and as a commercial bleaching and sanitizing agent. Chemically, these commercial compositions usually contain a major portion of Ca(OCl).sub.2 with the remainder being a mixture with varying amounts of sodium chloride, calcium chloride, calcium hydroxide, calcium carbonate, calcium chlorate and other chemicals, and having a water content that may range from less than about 1% by weight up to about 15% by weight. However, such products are always nominally referred to in the industry as solid calcium hypochlorite compositions. As particular examples, see U.S. Pat. Nos. 2,963,440, issued to Robson on Dec. 6, 1970; 3,036,013, issued to Jaszka et al on May 22, 1962; 3,544,267, issued to Dychdala on Dec. 1, 1970; 3,560,396, issued to Robson on Feb. 2, 1971; 3,669,894, issued to Faust on June 13, 1972; and 3,793,216, issued to Dychdala et al on Feb. 19, 1974.
A great number of processes are known in the art for manufacturing these solid calcium hypochlorite compositions. U.S. Pat. No. 3,134,641, issued to Robert D. Gleichert on May 26, 1964, discloses the method of reacting hypochlorous acid (HOCl) with slaked or hydrated lime at a pH of about 9.5 followed by drying the reaction medium to form the product. U.S. Pat. No. 3,241,912, issued to Nicolaisen on Mar. 22, 1966, discloses the chlorination of a mixture of calcium hydroxide and sodium hydroxide in a specific column apparatus to form calcium hypochlorite product. U.S. Pat. No. 3,544,267, issued to Dychdala on Dec. 1, 1970, teaches a method wherein a mixture of hydrated lime and caustic are chlorinated at low temperatures, i.e., 12.degree. C, to form a slurry containing triple salt -- Ca(OCl).sub.2.NaOCl.NaCl.12H.sub.2 O --, this triple salt is separated from the solution and added to a chlorinated lime slurry to convert the NaOCl contained in the triple salt to Ca(OCl).sub.2, and the resulting product mixture is dried to form a desirable stable calcium hypochlorite composition. U.S. Pat. No. 3,895,099, issued to Sakowski on July 15, 1975, teaches still another technique. In this method, an aqueous solution of caustic is chlorinated to form a sodium hypochlorite solution containing solid sodium chloride, the solid sodium chloride is then removed and hydrated lime is mixed with the sodium hypochlorite solution. This aqueous slurry of hydrated lime and sodium hypochlorite is chlorinated and the resulting product mixture is dried to form a stable, solid calcium hypochlorite composition.
Numerous variations and modifications of each of these processes have been practiced, but normally in almost every case, an aqueous slurry containing hydrated lime; a mixture of hydrated lime and caustic; or a mixture of hydrated lime and sodium hypochlorite is chorinated and this resulting reaction product is further processed, normally including a subsequent drying step, to form a solid calcium hypochlorite composition.
In the past, the calcium hypochlorite industry has generally always been able to utilize relatively pure hydrated lime, i.e., high in Ca(OH).sub.2 with minor, but tolerable levels of impurities, to make their composition products. In those cases where relatively impure hydrated limes were employed, the lime impurities were removed during processing, often preliminarily to the actual making of the calcium hypochlorite solution. Note that U.S. Pat. No. 3,895,099 teaches to remove these impurities in the very first step. These lime impurities may include insoluble impurities such as silica, aluminum salts, iron salts, magnesium salts, magnesium oxide, magnesium hydroxide, unburned limestone (calcium carbonate and magensium carbonate) and other compounds in trace quantities. However, like many industrial raw materials today, supplies of relatively pure hydrated lime are diminishing. Therefore, in the future, manufacturers may have to employ hydrated limes that contain more impurities. With the use of these relatively more impure hydrated limes, new problems associated with the impurities which in the past were or could have been overlooked because of the minor amounts then involved, now must be recognized and solved.
In particular, many impure hydrated limes contain relatively large amounts of magnesium in the form of magnesium salts, magnesium oxide and/or magnesium hydroxide, especially the latter two compounds. However, magnesium has been well known in the art to be associated with instability problems of calcium hypochlorite composition products and, therefore, hydrated limes containing relatively large amounts of magnesium were in the past not desired by the calcium hypochlorite industry. In particular, it was found that the chlorination of magnesium-containing compounds especially MgO and Mg(OH).sub.2 will result in the formation of magnesium hypochlorite Mg(OCl).sub.2. This latter compound will decompose readily and is very unstable under many conditions, especially in the absence of water. Therefore, during the subsequent drying step, or steps, for making calcium hypochlorite compositions, Mg(OCl).sub.2 has a good chance to decompose. This decomposition gives off a large exothermic heat of reaction which may cause fires in the dryers. Further, this heat of decomposition may also cause the calcium hypochlorite admixed therewith to decompose and, thereby, cause the final product to have lower amounts of Ca(OCl).sub.2.
On the other hand, a good supply of high magnesium-containing hydrated lime exists and could represent a new source for the manufacture of solid calcium hypochlorite compositions. Therefore, it would greatly benefit the industry if a new process could be developed or present processes improved whereby hydrated limes having relatively high levels of magnesium present therein could be utilized.